"Her voice was ruined, and her comeback wasn't what it was supposed to be, because her voice wasn't what it used to be," a close pal of Houston's tells PEOPLE.
The Grammy-winner and actress, 48, was set to make her return to the big screen. Houston recently filmed Sparkle, due to arrive in theaters Aug. 10, with Jordin Sparks.
But being in the spotlight was difficult for Houston, according to the source, who says she "was in pain from all the pressure she was facing ... I know from when she was recording her most recent album – her comeback album (I Look to You in 2009) – her voice was shot. That bothered her."
As the source notes, "In the music industry, you're only as good as your last hit, even if you are Whitney Houston. Then she had people gossiping about her ... It was a lot to deal with."